Pulmonary tissue which has received an acute toxic insult may either repair itself normally or develop a chronic fibrotic lesion characterized by the deposition of excess collagen. The inhibition of repair after the induction of acute lung damage enhances the development of fibrosis. Corticosteroids, which are the most widely used drug to treat patients with damaged lung tissue, have been reported to inhibit lung repair. This project will investigate the potential for fibrosis to develop in animals treated with corticosteroids after the induction of lung damage with butylated hydroxytoluene (BHT). Further studies will examine the effect of corticosteroids on collagen synthesis and degradation in damaged lung tissue. The rate at which collagen is synthesized in lung tissue in vitro is proportional to the extent of the initial damage. The relationship of these in vitro rates to the in vivo situation is not known. This project will compare rate of pulmonary collagen synthesis in vitro and in vivo. Collagen synthesis will be assessed by measuring the conversion of (3H) proline to (3H) hydroxyproline and by the specific digestion of labeled collagen by collagenase. These comparisons will be made in normal lung tissue and during the development of various levels of pulmonary fibrosis. Fibrosis will be generated by treating mice with BHT and subsequently exposing them to hyperoxia. Demonstrating that in vitro assays of the rate of lung collagen synthesis accurately reflect the in vivo situation will support the use of the more convenient in vitro assay to quantitate small amounts of lung damage. Additional experiments will assess the role of degradation of newly synthesized and established collagen in collagen homeostasis and pulmonary repair processes after acute lung damage and corticosteroid therapy. The degradation of newly synthesized collagen will be determined by measuring acid-soluble (3H) hydroxyproline following the administration of (3H) proline. The degradation of established collagen after various doses of BHT will be determined by measuring the rate at which (3H) hydroxyproline deposited following the intratracheal administration of (3H) proline is lost from lung tissue. The types of collagen synthesized at various times after the induction of lung damage and corticosteroid therapy will be determined by using an HPLC technique to separate collagen peptides following a cyanogen bromide digestion.